Artillery shells were fired toward South Korean-controlled Yeonpyeong Island, Yonhap News Agency quoted the officials as saying, adding that the shells fell in waters north of the Northern Limit Line (NLL), the de facto inter-Korean maritime border.

CNN said the North fired artillery shells on Wednesday in the same area, saying they were part of an annual training drill.

"We have confirmed North Korea's firing of several artillery shells, but they did not cross" the two countries' maritime border, said Park Sung-woo, of Seoul's joint chiefs of staff, according to Yonhap. "We are on high military alert."

"Following the firing by North Korea, South Korea responded by shooting Vulcan canons into the air, a statement that it would not be intimidated by saber-rattling by the communist neighbor," Yonhap said.